1. Field of the Invention
The present invention relates to an electrostatographic encapsulated toner employable for producing a visible image from a latent image in a recording method utilizing an electrostatography.
2. Description of Prior Arts
As the process for fixing a toner image in a recording method such as an electrostatography, there have been known three fixing processes, that is, a heat fixing process, a solvent fixing process and a pressure fixing process. Recently, the heat fixing process and the pressure fixing process, both using no solvent, are widely used from the viewpoint of the prevention of environmental pollution.
In the heat fixing process, a toner comprising a colorant bound with a binder has been conventionally employed. The same kind of toner is also employed in the pressure fixing process, but utilization of an encapsulated toner is recently proposed in the pressure fixing process.
The encapsulated toner is a toner in the form of micro-capsule prepared by enclosing a core material comprising a colorant such as carbon black and a binder with a resin shell which is rupturable by the application of pressure.
The conventional encapsulated toner is not necessarily satisfactory in various properties that are essentially required for a toner.
As a developing agent employable for the electrostatography, there has been conventionally employed a toner which has insulation properties on its surface, but recently a toner provided with an appropriately positive or negative friction-electrostatic chargiablity on the surface has been developed. This electrostatography comprises procedures of electrostatically charging a toner by rubbing the toner with carrier particles such as iron powder or furs, and causing a latent image to draw the electrostatically charged toner. Accordingly, the toner employable in the electrostatography is required to have positive or negative friction-electrostatic chargiablity corresponding to the negative or positive electrostatic charge of the latent image depending on the nature of an apparatus employed.
As a method for giving friction-electrostatic chargeability to the conventional toner, there is generally utilized a method of adding an electrostatic charge modifier to a toner in combination with a resin to disperse the electrostatic charge modifier inside the toner in the procedure of the preparation of a toner. However, as for an encapsulated toner, it is difficult to effectively fix the electrostatic charge modifier onto the surface of encapsulated toner, and accordingly a powdery electrostatic charge modifier such as silica powder is generally deposited onto the surface of the encapsulated toner so as to give the friction-electrostatic chargeability to the encapsulated toner.
However, the present inventor has found that in the encapsulated toner deposited with an electrostatic charge modifier such as silica powder on its surface, the electrostatic charge modifier easily separates or drops off from the encapsulated toner when the encapsulated toner is stored or employed for a long period of time. In the case that a relatively large amount of the electrostatic charge modifier separates or drops off from the encapsulated toner, the encapsulated toner tends to be insufficient in the developing properties so as to cause unfavorable effect on the image characteristics of the resulting visible image such as lowering of sharpness or density. The encapsulated toner is also needed to be excellent in other various properties such as powder flowability and preservation stability than the above-mentioned friction-electrostatic chargeability. If any one of those properties is deteriorated, the resulting visible image having high image quality can be hardly obtained.
For providing a positive friction-electrostatic charge to an encapsulated toner, there has been proposed a method of forming a resin shell from polymers obtained from nitrogen-containing monomers such as N-vinyl compounds, vinyl imidazole or derivatives thereof, vinyl pyridine or derivatives thereof, acrylamide or derivatives thereof, compounds containing other vinyl group or amino group, and quaternary ammonium salts of those compounds or derivatives, as described in Japanese Patent Provisional Publications No. 59(1984)-185353, No. 59(1984)-187350, No. 59(1984)-187352, No. 59(1984)-187355, No. 59(1984)-187357, No. 59(1984)-189354, No. 59(1984)-189355.
The above-mentioned publications describe the use of a solid binder such as a polymer easily interlocking with fibers of ordinary papers, for instance, polyethylene or a paraffin wax, as a binder employable for the encapsulated toner.
However, it has been confirmed by the present invention that while the encapsulated toner comprising a resin shell of the above-mentioned polymers of the nitrogen-containing monomers and the solid binder such as a polymer or a paraffin wax is favorable in the friction-electrostatic positive chargeability, such toner is insufficient in the pressure fixability. Accordingly, the employment of such encapsulated toner in the electrostatography utilizing a pressure fixing process requires very high pressure for the fixing, and such a high pressure causes various problems such as requirement of a great-size apparatus, occurrence of fatigue of paper sheets employed for receiving the toner image, and occurrence of glossy surface on the paper sheet. Further, in the case of using such toner containing the solid binder, on the surface of a paper sheet having a toner image fixed thereon are placed a relatively large amount of toner particles in the protruded form, and hence the toner image tends to be moved or stained by mechanical shock such as friction between the paper sheet having the toner image thereon and other paper sheets or fingers.
An oily composition comprising a solid polymer and a high-boiling organic solvent capable of dissolving or swelling the solid polymer (e.g., solvent having a boiling point of not lower than 150.degree. C.) is also known as a binder of an encapsulated toner employable in the pressure fixing process. Accordingly, it can be presumed that the combination of the aforementioned resin shell having excellent friction-electrostatic chargeability of positive charge and the above-mentioned oily binder is utilized. However, it is very difficult to densely form a resin shell of the aforementioned polymer which has a nitrogen-containing monomer as a main monomer component around the oily binder.